Urine markers in obese patients signal stone risk

November 1, 2006

Cleveland-A Harvard Medical School study published last year in JAMA (2005; 293:455-62) established a link between obesity and kidney stone formation. Now researchers at the University of Minnesota have taken the next step with a study showing that obese men and women have at least one, if not more, urine abnormalities that increase their risk of stone formation.

"There have been a number of studies that [have] looked into databanks of people with stones and that have suggested that there are differences in urinary variables between obese people who present with stones and those who present with stones but are not obese. Those have been retrospective studies. This is the first study that we know of to look prospectively at the baseline urinary parameters of obese patients without a history of stone disease," Manoj Monga, MD, professor of urologic surgery at the University of Minnesota, Minneapolis, told Urology Times.

The study presented here at the World Congress of Endourology focused on a special cohort of obese patients: those scheduled for bariatric surgery.

To derive baseline parameters, Dr. Monga and his colleagues identified nine women and four men scheduled for bariatric surgery. Their mean age was 47 years; their mean body mass index was 53. The National Institutes of Health classifies a BMI ≥40 as morbidly obese. The patients in this study were required to collect their urine over a 24-hour span and to maintain a 24-hour dietary intake diary.

Elevated stone risk

"The first thing that jumped out of this study was that every one of these patients had at least one abnormality that puts them at risk for forming stones," Dr. Monga said. "The most common abnormality was low volume. The second most common was high sodium."

Study data found that 75% of the patients had low urine volume, and that 43% had hypernatriuria. The mean total urine volume was 1.3 L/day. Seventeen percent had hyperoxaluria, and 17% had hypocitraturia.

"Establishing the baseline prevalence of hyperoxaluria and hypocitraturia in the morbidly obese is of particular importance, as these are two abnormalities commonly found in stone patients after bariatric surgery," Dr. Monga said.

Elevated relative super saturation was identified for uric acid in 60% of the patients, for sodium urate in 52% of the patients, for calcium oxalate in 35% of the patients, and for brushite in 13% of the patients. Low urinary pH was seen in 38% of the patients.

"The next step will be to expand this database up to at least 50 patients and follow them after surgery to see what parameters change and how much. That will hopefully lead to a plan or algorithm for bariatric surgeons and perhaps answer questions such as whether candidates for bariatric procedures should be screened and treated with dietary interventions to prevent stone treatment," Dr. Monga said.

A number of physicians supported the need for such research, noting the epidemic of obesity, the increasing numbers of bariatric surgeries, and the dearth of current information relating to obesity, bariatric procedures, and stone formation.

Data from the NIH show that 129.6 million U.S. adults, comprising two-thirds of the population, are overweight (BMI >25) and that 61 million-one-third of the adult population-are obese (BMI >30).